Birth control pills can limit muscle-training gains

Some female athletes may pay a price for using oral contraception: lower strength gains from resistance exercise. These muscle-building exercises depend on lifting weights (like barbells and those in the big machines at your local gym) or working against tension bands and bars (like those in Bowflex devices).

Personal trainers have long noted that all women don’t garner the same benefits from such exercise and there has been a suspicion that genetics might underlie differences. But exercise physiologist Chang Woock Lee and his colleagues at Texas A&M University wondered if other lifestyle factors might also play a role. And at the Experimental Biology meeting in New Orleans, this week, Lee identified birth control pills as a major suspect.

In an earlier study, Lee’s group noted that many young female athletes reported using oral contraception. These pills have been specifically formulated to alter a woman’s steroid-hormone levels. Since certain steroids can affect how efficiently the body bulks up and gains muscle, Lee wondered whether these pills might also limit strength gains.

So, three times a week for 10 weeks, the researchers had 73 young women (18 to 34 years old) complete 13 different exercises. The regimen was intense, working muscles throughout the body. None of the recruits had been regularly working out beforehand. But they sure were now. Each had to complete her resistance training against weights that were individually tailored to work her muscles at 75 percent of their maximum strength.

Thirty-four of the recruits were taking some type of oral contraceptive. And as a group, they gained 2.1 percent muscle mass over the trial — or 40 percent less than the group not on the pill. So I asked Lee: What do you think is going on?

One clue: Both before and after the trial, he found, women using oral contraception had dramatically lower blood levels of natural anabolic — as in muscle-building — hormones than did recruits not on the pill. The anabolic hormones included DHEA and its more abundant sulfated form, DHEAS. Compared to non-pill-users, women taking oral contraceptives also had substantially higher concentrations of cortisol, a hormone associated with the breakdown of muscle.

So resistance training “did not rescue” this adverse hormone profile in the pill users, Lee points outs. And, he suspects “those hormone differences might be the underlying factor” accounting for why women on the pill gained less muscle.

But that’s not the end of the story. Further analysis showed that not all pill users had an equally hard time gaining muscle through exercise. So Lee analyzed the formula for each woman’s contraceptive and found that many types contained progestins — a synthetic form of progesterone, a natural female sex hormone. In tests, he showed that these synthetic hormones bind to the androgen receptor in cells — the same receptor that binds male sex steroids, such as DHEA and DHEAS.

Lee worries that if progestins dock with a receptor that DHEA would normally have turned on, they could essentially block the true androgen's ability to trigger muscle growth.

Not all progestins were equally effective at this binding, however. So Lee grouped the contraceptives on the basis of whether their progestins exhibited low, medium or high androgenicity (i.e. androgen-receptor binding). And then a really striking difference emerged.

Women who took pills containing low-androgenicity progestins acquired muscle through exercise as effectively as did women not on the pill. But women whose oral contraceptives contained medium- or highly androgenic progestins attained less than a 0.5 percent gain in muscle mass over 10 weeks — a far cry from what the others achieved. (By the way, Lee didn’t divulge which brands of contraceptives performed which way, so please don’t write me asking.)

What all this means, he says, is that for women taking certain brands of the pill, the difference in strength-building from exercise could “make a difference in the color of her medal.” For the rest of us, he says, getting stronger would just require that we work “much harder.”